The present invention relates to a broad band, high sensitivity, alternating magnetic field sensor making it possible to obtain a high-performance measuring apparatus.
The development of electromagnetic transmission has led to a widening of the frequencies, both on the low frequency side (a few dozen Hz for submarine transmissions) and on the high frequency side (several dozen gigahertz for satellite transmissions).
Most of the magnetic field sensors only cover in each case a reduced frequency range, which is a few octaves in the best of cases.
Therefore, the interest of a very wide band sensor making it possible on the one hand to replace several narrow band sensors and on the other to detect a phenomenon, whose instantaneous frequency spectrum is very broad is obvious. This type of sensor also makes it possible to study transient phenomena (interference, atmospherics, lightning, etc.), whose frequency spectrum is not always also known a priori, so that it is necessary to seek the phenomenon in the widest pass band possible.
Magnetic field measurements can be carried out with sensors using several techniques, including the Hall effect and the induction of a voltage in a conductor in accordance with the Lorenz law. Hall effect sensors are limited to frequencies of a few megahertz. However, the frequency limitation of sensors using magnetic induction only results from the construction and geometry of the sensor.
The voltage induced in a coil is proportional to the frequency of the phenomenon, the total cross-section of the coil and to the number of turns. In order to obtain a good sensitivity at low frequencies, it is therefore necessary to increase the cross-section, as well as the number of turns. However, to widen the response to high frequencies, it is necessary for the coil to have low stray capacitances and consequently a reduced number of turns, and that the dimensions of the sensor are small compared with the wavelength corresponding to the highest frequency. Moreover, in order to obtain a good rejection of the electric field E, the coil must be shielded, which increases stray capacitances and to the same extent limits the frequency response. It is therefore difficult to construct a sensor having a wide pass band and a good sensitivity at low frequencies.
The shielded frame is a known solution in the form of a shielded flat coil. The shield is slotted, so that it does not form a short-circuited loop, which would absorb virtually all the magnetic field by induced eddy currents. Such a magnetic field sensor can be optimized either for a good sensitivity at low frequencies (large diameter, large number of turns), or for a response at high frequencies (small diameter, small number of turns). The first optimization leads to a poor response at high frequencies (excessively low resonant frequency), whilst the second optimization leads to a limitation of the sensitivity at low frequencies (excessively reduced overall cross-section of the coil).
The Moebius loop is another known solution, which comprises two half-loops produced with a coaxial cable, which are connected in series by connecting the central conductor of one of them to the sheath of the other and vice versa. This arrangement has a much broader response to high frequencies than that of a conventional shielded frame. As the structure is coaxial and matched at the output, the limitation at high frequencies does not appear, provided that the wavelength remains much higher than the diameter. The sensitivity at low frequencies corresponds to that of a frame with the same dimensions and having two coils.
Thus, neither of the two known devices makes it possible to obtain both a broad frequency response and a good sensitivity at low frequencies.